Give the application a settings area built from what really exists

src/DodoSSH.Client.App/Views/SettingsSecurityPage.axaml.cs

@@ -0,0 +1,9 @@
+using Avalonia.Controls;
+
+namespace DodoSSH.Client.App.Views;
+
+/// <summary>Settings mode's Security page: the E2E explainer, Windows Hello, and approved host keys.</summary>
+internal sealed partial class SettingsSecurityPage : UserControl
+{
+    public SettingsSecurityPage() => InitializeComponent();
+}